In a substrate processing apparatus for performing plasma etching or the like on a semiconductor wafer, components made of various materials are provided in a processing chamber where the semiconductor wafer is accommodated. Among the components, a focus ring or an electrode plate forming a shower head is worn by a plasma. Therefore, there are suggested various techniques that measure a wear amount of a component to check replacement timing of the worn component.
For example, a reference piece having a known thickness is thermally coupled to an unused focus ring having a known thickness by a structure that is not substantially worn by a plasma. Then, the focus ring is installed at the substrate processing apparatus. Low-coherence light is irradiated to the focus ring from a bottom surface side of a wear amount measurement target position (hereinafter, referred to as “measuring position”) of the focus ring and also irradiated to a reference mirror and the reference piece. Thereafter, there is measured an interference waveform between reflected light from the reference mirror in the case of moving the reference mirror in a direction parallel to an incidence direction of the low-coherence light, reflected lights from a top surface as a wear surface and a bottom surface of the focus ring, reflected light from the reference mirror, and reflected lights from a top surface and a bottom surface of the reference piece.
A thickness of the reference piece can be obtained from the interference waveform between the reflected light from the reference piece and the reflected light from the reference mirror and the moving distance of the reference mirror. A thickness at the measuring position of the focus ring can be obtained from the interference waveform between the reflected light from the focus ring and the reflected light from the reference mirror and the moving distance of the reference mirror. A thickness of the focus ring can be obtained from a ratio between the measured thicknesses and the known thickness of the reference piece (see, e.g. Japanese Patent Application Publication No. 2011-210853).
However, the method disclosed in Japanese Patent Application Publication No. 2011-210853 is disadvantageous in that the cost of the focus ring is increased because the reference piece needs to be coupled to the focus ring and also in that the measurement time is increased because the reference mirror needs to be moved.
To that end, there is suggested a method that measures a thickness of a component to measure a temperature of the component by using a optical frequency domain spectral interferometer (see, e.g., Japanese Patent Application Publication No. 2013-029487). In the method disclosed in Japanese Patent Application Publication No. 2013-029487, a thickness of the component at an irradiation position is measured by performing Fourier transform on spectrum distribution of reflected light from a top surface of the component and reflected light from a bottom surface of the component in the case of irradiating low-coherence light to a predetermined position of the component.
In the method disclosed in Japanese Patent Application Publication No. 2013-029487, a temperature of the component is measured by using previously obtained data on relationship between a temperature and a thickness of the component without measuring a wear amount of the component. However, in the case of measuring a thickness of a component by using the method disclosed in Japanese Patent Application Publication No. 2013-029487, a measurable thickness is determined by resolution of a spectroscope for detecting reflected light. For example, if a central wavelength is denoted by λ0; a wavelength band is denoted by Δw; and the number of photodetectors of the spectroscope (the number of CCD devices) is denoted by N, the relationship between the parameters and a maximum measurable thickness x is expressed by the following equation 1.
                    x        <                              λ            0            2                                2            ⁢                          (                              Δ                ⁢                                                                  ⁢                                  ω                  /                  N                                            )                                                          Eq        .                                  ⁢        1            
For example, when λ0=1550 nm, Δw=40 nm and N=512, x becomes 15.38 mm. When the component is made of Si, a maximum measurable thickness x0 may be calculated by dividing x by 2n (n being a refractive index (about 3.65)). Therefore, x0=x/2n==2.1 (mm) is obtained. In other words, if the component made of Si has a thickness greater than about 2.1 mm, the thickness thereof cannot be measured by the method disclosed in Japanese Patent Application Publication No. 2013-029487. As a result, a wear amount of the component cannot be measured.